An electrochemical accumulator usually has a nominal voltage of the following magnitude:                1.2 V for NiMH type batteries,        3.3 V for lithium ion iron phosphate or LiFePO4, technology,        3.7 V for cobalt-oxide-based lithium ion type technology.        
These nominal voltages are too low for the requirements of most systems to be powered. To obtain the appropriate level of voltage, several accumulators are connected in series. To obtain high values of power and capacitance, several groups of accumulators are placed in series. The number of stages (number of groups of accumulators) and the number of parallel-connected accumulators in each stage varies according to the voltage, the current, and the capacitance desired for the battery. The association of several accumulators is called a “battery of accumulators.”
The charging of an accumulator results in an increase in the voltage at its terminals. Each accumulator technology has a charging profile that is proper to itself, defined for example by the development in time of the voltage of an accumulator for a given charging current.
An accumulator is considered to be charged, for example when, under a given current, it has reached a nominal voltage level defined by its electrochemical process. If the charging is interrupted before this voltage is reached, the accumulator is not fully charged. The accumulator can also be considered to be charged when the charging has lasted for a predetermined time or again when the charging current, with the accumulator being under constant voltage, has reached a minimum threshold value.
Owing to manufacturing variations, accumulators have different characteristics in practice. These differences, which are relatively low when the battery is new, are accentuated with the heterogeneous wearing out of the accumulators of the battery. Dispersions persist even when accumulators from a common manufacturer are associated in a battery. A control device, which uses voltage measurements from the different accumulators, generally supervises the charging of the battery.
The range of voltage of a cobalt-oxide-based Li-ion type accumulator typically ranges from 2.7 V to 4.2 V. Use outside this range can induce an irreversible deterioration of the accumulators of the battery. Excess charging can lead to destruction of an accumulator, accelerated wear by deterioration of its electrolyte, or an explosion by thermal stalling. Prior-art control devices thus monitor the charging of each accumulator. The charging of all the accumulators is therefore interrupted when the most highly charged accumulator reaches a top limit of its range of operation. The voltage of the least charged accumulator is then equal to a voltage lower than the top limit.
The control device also interrupts the discharging of the battery when the least charged accumulator has reached a low limit of the range of operation.
There are therefore various known connectors to enable the control device to verify the level of charging of each of the accumulators. To increase the level of security of the battery, it is also a frequent practice to use connectors enabling the control device to verify other working parameters such as the temperature of the accumulators.
In a known structure, when several accumulators are present, a circuit for measuring voltage and temperature is fixed onto each accumulator. The control device comprises several slave boards managed by a master board or a computer, these boards being grouped together. Each slave board is connected to several measuring circuits, for example 8 or 16, by means of point-to-point wiring.
In such a structure, the accumulators are at scaled voltages attaining high levels. Thus, the voltage measurements must be either galvanically isolated or designed for a high common mode voltage. In a motor vehicle, the computer is generally powered by a 12 V battery dedicated to the powering of the embedded network and its accessories. Since the battery of the embedded network is connected to the ground of the vehicle, it can furthermore prove to be necessary to set up galvanic isolation for communication between the slave boards and the computer. In addition, the point-to-point wire connections between the measuring circuits and the slave boards require a large number of connections and substantial wiring length. Such a design therefore gives rise to a cost and complexity that are non-negligible. Besides, the number of point-to-point wire connections multiplies the risks of shorting with a direct connection to the accumulators. This calls for particularly careful design and manufacturing that imply especially the integration of the protection systems (fuses or circuit breakers).
The document JP2009-089453 describes a battery provided with several series-connected accumulators, a circuit for measuring voltage at the terminals of the battery, a communications circuit and a control circuit. The communications circuit informs the control circuit about a variation in voltage when the voltage at the terminals of the battery crosses a threshold.
According to another known structure, when there are numerous accumulators present, a circuit for measuring voltage and temperature is fixed onto each accumulator. The measuring circuits are connected to a same communications bus. A computer will retrieve the information transmitted by measuring circuits on this communications bus.
Such a structure avoids the drawbacks of point-to-point wire connections. However, such a structure requires the presence of a bus of particular design having galvanic isolation owing to the voltage levels to which it is subjected. Very special attention must be paid to the design of the bus, since it is connected to very high voltage levels within the battery. Thus, a costly and complex galvanic isolation has to be implemented and the integration of such a bus in a battery raises practical problems.